The acoustic impression of a motor vehicle is determined for the most part by the exhaust noise, that is, the sound waves emerging from the outlet of the exhaust system. In modern motor vehicles, in particular those having diesel engines, the noises and oscillations that are originally generated by the engine and are directed into the exhaust system are greatly affected and damped by exhaust gas purification systems such as catalytic converters or particulate filters. The exhaust noise therefore no longer corresponds to the natural, undamped engine noise; rather, it sounds damped and not very sporty. Also in the case of smaller engines such as four-cylinder engines, the exhaust noise sounds little attractive.
To provide a remedy and to give such vehicles a sportier sound, exhaust systems are known in which a loudspeaker is in fluid communication with the exhaust branch, so that it can emit sound waves into the exhaust branch. This allows the exhaust noise to be influenced by a selective control of the loudspeaker.
Usually, an attempt is made to generate an exhaust noise that consists of two components.
The first component of the exhaust noise consists of harmonic oscillations generated by the engine. The fundamental order of the harmonic oscillations has the frequency of half the rotational frequency of the engine and the higher orders are multiples thereof. Accordingly, the frequency of these harmonic oscillations is dependent on the speed of the engine.
In FIG. 1, the frequencies of three orders are schematically plotted at different speeds for an exemplary 4-cylinder engine. Only these three orders are present at the end of a common exhaust system, that is, at the outlet of the tailpipe into the environment. They are shown as solid lines in FIG. 1. The sound of these three harmonic oscillations is unnatural and not very sporty.
For this reason, it is known to insert harmonic oscillations that correspond to further orders, in particular intermediate orders, into the exhaust branch through the loudspeaker. They are illustrated as dashed lines in FIG. 1. An exhaust noise generated in this way sounds considerably more attractive and more high-quality.
The second component of the exhaust noise is the so-called static noise. Static noise results from the steady component of the flow velocity of the exhaust gas. High flow velocities cause swirls in the exhaust branch, which, in turn, are perceived as noise. The steady component of the flow velocity increases as the quantity of exhaust gas produced by the engine—hence, the engine speed—increases, and will be perceivable as of a particular speed. The static noise can also be generated or increased by the loudspeaker so as to obtain as natural an acoustic pattern as possible.
However, particularly at low engine speeds, even an exhaust noise influenced in this way does not yet correspond to the natural exhaust noise of the engine used or that of a more sporty engine the sound of which one tries to simulate.
It is the object of the invention to provide a method of influencing the exhaust noise of a motor vehicle and also an exhaust system for a motor vehicle which further enhances the impression of a natural and/or sporty exhaust noise.